The present invention relates to time delay integration of focal plane photodetector signals and, more particularly, to photodetector signals formed in the focal plane and transmitted to a monolithic integrated circuit for signal processing.
Acquiring a representation of a field of view at a location remote from the user, or at wavelengths differing from those a user can sense directly, have been goals which have led to the development of electromagnetic radiation sensing and signal processing systems. In other situations, such systems are provided primarily for recordation of fields of view, or primarly to permit manipulation of the field of view images. There are other purposes, and in many systems a combination of these goals are desired.
Various kinds of electromagnetic radiation sensing and signal processing systems have been used for these purposes. Two-dimensional arrays of photodetectors have been used as a basis for sensing over the lateral extent of an image resulting from a field of view. Such systems require large numbers of photodetectors and so are expensive and difficult to implement. An alternative photodetector arrangement is formed by having a single line of photodetectors, and a scanning system which scans in orthogonal directions to cause successive portions of images from a field of view to sequentially impinge on the line of photodetectors. Such systems add the complexity of a mechanical scanning arrangement to cause such image portions to be sequentially presented to the photodetectors, but they also conveniently permit the use of the time-delay integration technique to improve signal-to-noise ratios. An intermediate possibility is to form many lines of photodetectors so that scanning in only one direction need be provided to ease the mechanical complexity but still provide use of time-delay integration.
The time-delay integration technique is especially important in infrared radiation detection. This is because impinging infrared electromagnetic radiation from a field of view usually has the portion therein containing the signal information needed for differentiating objects in that field of view from the general background radiation portion therein that is a small fraction of that background radiation portion, i.e. the useful signals are small so that noise is a significant problem. In this technique, a line portion of an image from the field of view is swept across the line of photodetectors sequentially so that each resolvable element in the image line portion passes over each of the photodetectors in the line thereof in sequence. The output signal from each photodetector for the time a particular resolved element of the image line is thereover is collected to provide a combined signal for that element based on the contributions of each detector. Combining the photodetector signals into the combination signal causes the information portion of the scene element from each photodetector to add directly, i.e. proportionate to the number of detectors in the line, but the noise portion of each element from each photodetector adds in quadrature to be proportional to the square root of the number of detectors to thereby improve the signal-to-noise ratio.
Collecting the output signal from each photodetector at the time a resolvable element in the line image is impinging thereon in effect requires that the rate of sweeping the line image across the photodetectors be matched by the rate at which information from each detector, or information subsample, is collected. Thus, the scanning of the image line must be synchronized with the collecting and combining of subsamples from each of the photodetectors in the line thereof to form a final complete sample for a resolvable element from the image line.
An arrangement used for accomplishing such collection of samples has been to have each photodetector connected to a well in a charge-coupled device so that the charge from the photodetector collected over a period of time can be transferred to the charge-coupled device well. Then, if the transfer rate of charge packets from well to well in the charge-coupled device is synchronized with the line image scan rate across the photodetectors, the charge accumulated in each photodetector for a resolvable element in the line image impinging thereon can be collected in the associated charge-coupled device well after the time that the resolvable element has impinged on the photodetector. This charge is transferred along the charge-coupled device line to the next well connected to the next well connected to the next photodetector on which the resolvable element impinges from which the resulting charge is to be added to the previously collected charge. Thus, at the last photodetector in the line thereof, the corresponding charge-coupled device well will contain the charge supplied from each photodetector due to that particular resolvable element from the line image having been swept over the photodetectors to impinge successively on each. This total collected charge can then be transferred to an output arrangement as the full sample taken of that resolvable element.
Such a system has some drawbacks. Assume, as is typical, that the photodetector elements are formed in a separate material, typically HgCdTe photoconductive sensors, separated from the silicon monolithic integrated circuit in which the charge-coupled devices are formed. Then there must be a separate interconnection from each photodetector to the storage site used in the silicon monolithic integrated circuit to store the charge generated by the electromagnetic radiation impinging on that photodetector by which it is transferred to the charge-coupled device. Thus, there will be a number of interconnections are required between the structure containing the photodetectors and the integrated circuit chip structure containing the charge-coupled device structures.
The charge-coupled device structures must be fabricated in the silicon monolithic integrated circuit immediately adjacent to the storage sites to which the leads from the photodetector structure are connected to permit transfers from these sites to the charge-coupled device. Thus, significant portions of the processing circuitry must in effect be provided in the same area of the silicon monolithic integrated circuit corresponding to the associated photodetector. This limits the size of the storage sites which can be provided for collecting charge from the photodetector induced by electromagnetic radiation impinging thereon. Thus, there will be a limit on the density of photodetectors which can be used or a limit on the amount of charge which can be collected from a photodetector during use, or both. This is particularly a problem for the later wells in the charge-coupled device line thereof which are accumulating the increasingly greater combined charge for a resolvable element in the image line from successive photodetectors.
Keeping the amount of charge which can be collected from each photodector during use small so that the total does not get too large for the charge-coupled device wells requires that the amount of time that charge is accumulated on a photodetector by the resolvable element be kept relatively short. Thus, the operating speed must be increased, and the small charge levels lead to noise components being relatively large with respect to the photodetector signal. In addition, the fabrication of charge-coupled devices in a monolithic integrated circuit which has other kinds of circuitry also provided therein, typically circuits based on metal-oxide-semiconductor field-effect transistors (MOSFET's), require an additional masking step during fabrication. This increases cost, and limits the number of vendors which can supply such integrated circuits as many will not have a process which provides for the additional masking and fabrication step. Thus, there is a desire to provide a signal processing means for photodetector signals which does not lead to limits on the photodetector charge packet signal size, and which can be fabricated in common MOSFET fabrication processes.